Electronically controlled motors (ECM's), are commonly used, for example, in high-efficiency furnaces, air-conditioner blowers, and other airflow regulation applications requiring torque to be provided with a high degree of accuracy. As is well-known, the torque generated by a motor is the product of the current flowing in the primary winding of the motor and the back EMF provided by the motor. However, such motors are often driven by an inverter-switch motor drive, and motor torque can also be affected by the commutation angle and the conduction interval of the inverter switches of the motor drive. Nevertheless, because motor winding current can be accurately controlled to within a relatively small tolerance, and because the effect of the motor drive parameters on torque is substantially constant among motors operating at a given nominal speed, the dominant factor affecting torque output among similar motors is variance in the back EMF provided by each one.
The back EMF of a motor, in turn, depends on the magnetic strength of the permanent magnet used by the motor to create motive power. As will be appreciated by those of ordinary skill in the art, variations in magnet strength from one motor to another are inevitable due to inherent, slight differences in physical properties of the magnetic materials used for each motor, even among motors produced by the same manufacturing process. Further variations in magnet strength among motors can also be brought about by differences in ambient operating conditions (e.g., operating temperature) among otherwise identical motors.
To compensate for such variations in magnetic material strength among electronically controlled motors used for any particular application, each such motor must be independently calibrated to achieve a predetermined nominal level of output torque at a predetermined nominal operating speed. The nominal values for these parameters are often precisely specified for particular ECM applications.
Previously, calibration of an ECM has required the use of a closed-loop calibration station or dynamometer to measure the amount of torque produced by the ECM and raise or lower the supply current provided to the motor to maintain a desired torque level. This method of calibration via a dynamometer must be performed manually by an operator and is generally costly and time-intensive. Moreover, the dynamometer is a mechanical apparatus that is subject to wear and requires periodic maintenance which adds further to the time and expense associated with motor calibration by prior-art methods.